U.S. Pat. No. 2,646,344 to Kamlet discloses a single-step method of making boron phosphate. Phosphorus pentoxide and boric anhydride are reacted in equimolar amounts at between 577.degree. C. and 1000.degree. C. to form a solid white mass of boron phosphate that can be ground to a desired fineness.
U.S. Pat. No. 3,395,984 to Collins also discloses a single-step method of making boron phosphate. Equimolar amounts of phosphoric acid and ammonium biborate are heated rapidly to 200.degree.-350.degree. C. to form a dry, porous product.
A two-step method of making boron phosphate is outlined in Brauer, Handbook of Preparative Inorganic Chemistry, Vol. 1, p. 796 (1963). Equivalent amounts of boric acid and phosphoric acid are mixed and heated at about 100.degree. C. to form a paste which is then crystallized by firing for 2 hours at 1000.degree. C. Alternatively, monoammonium phosphate may be substituted for phosphoric acid in the reaction.